Climate and habitat type interact to influence contemporary dispersal potential in Prairie Smoke (Geum triflorum)

Abstract Understanding dispersal potential, or the probability a species will move a given distance, under different environmental conditions is essential to predicting species' ability to move across the landscape and track shifting ecological niches. Two important drivers of dispersal ability are climatic differences and variations in local habitat type. Despite the likelihood these global drivers act simultaneously on plant populations, and thus dispersal potential is likely to change as a result, their combined effects on dispersal are rarely examined. To understand the effect of climate and varying habitat types on dispersal potential, we studied Geum triflorum—a perennial grassland species that spans a wide range of environments, including both prairie and alvar habitats. We explored how the climate of the growing season and habitat type (prairie vs. alvar) interact to alter dispersal potential. We found a consistent interactive effect of climate and habitat type on dispersal potential. Across prairie populations, an increased number of growing degree days favored traits that increase dispersal potential or the probability of dispersing farther distances. However, for alvar populations, dispersal potential tended to decrease as the number of growing degree days increased. Our findings suggest that under continued warming, populations in prairie habitats will benefit from increased gene flow, while alvar populations will become increasingly segregated, with reduced potential to track shifting fitness optima.


| INTRODUC TI ON
Seed dispersal, or the movement of offspring away from the source parent plant, provides the opportunity for migration (Zobel et al., 2010), gene flow (Sexton et al., 2009), range shifts (Davis & Shaw, 2001;Hargreaves et al., 2015), and spatial tracking of favorable environmental conditions (Brehm et al., 2019;Edelaar & Bolnick, 2012)-all of which allow for species persistence despite local extirpation (Tilman, 1994).Indeed, dispersal evolves even in the simplest systems with homogenous environments (Hamilton & May, 1977) and is especially important for sessile organisms, such as plants, that have limited opportunities during their life cycle for movement (Beckman & Sullivan, 2023).In heterogeneous habitats, environmental conditions are an important driver of plant dispersal ability, which can manifest locally through differences in physical properties of the environment (e.g., soil depth, habitat structure, fragmentation) as well as in a more broad-scale way through the climate in the year of reproduction (Hernandez et al., 2023).When the impact of multiple environmental factors are considered synergistically on dispersal in theoretical studies, strong effects on plant movement are often found, with factors like fragmentation and climate change both slowing dispersal (Renton et al., 2012(Renton et al., , 2013)).
However, there is a dearth of empirical studies that directly examine how local environmental differences and broad-scale climate concomitantly alter plant dispersal.In an era of multiple anthropogenic global changes that simultaneously act on plant populations, it is imperative to determine how the local environment and climate singly and jointly influence plants' dispersal ability.Here, we focus on species that disperse their seeds by wind (anemochory), as those species are likely to be sensitive to environmental change (Beckman & Sullivan, 2023).
Climate is a key factors that influence seed dispersal (Hernandez et al., 2023), and an increasing thermal environment can have both positive and negative effects on dispersal capacity (Corlett & Westcott, 2013), with some plants actively controlling dispersal distance based on temperature (Seale & Nakayama, 2020).In some cases, warming temperatures and associated aridity can increase seed release height (Bjorkman et al., 2018;Teller et al., 2014), which has direct impacts on increasing potential dispersal distance (Thomson et al., 2011).This increase in dispersal can be exacerbated when these warmer temperatures change the speed, direction, and turbulence of wind conditions that further promote long-distance dispersal (Kling & Ackerly, 2020;Kuparinen et al., 2009).However, for some species, increasing temperature can have the opposite effect on dispersal.There is evidence that species that produce heteromorphic seeds will disproportionately produce seeds with reduced dispersal potential under warmer environments to persist if changed local environments are suitable, thus producing a larger proportion of seeds that travel shorter distances (Arshad et al., 2019;Lenser et al., 2016).To predict how dispersal might change as climate continues to warm there is a need to understand the degree to which species-specific dispersal potential is sensitive to climatic variation.
The local environment a plant grows in can also affect dispersal potential (Cheptou et al., 2008;Damschen et al., 2014;Hernandez et al., 2023).Deeper soils with more available nutrients can allow plants to grow taller (Bernard-Verdier et al., 2012;Dickson et al., 2014), which can increase dispersal ability (Thomson et al., 2011).When similar plant species exist in different habitat types, that vary in soil depth, as well as nutrient and water availability, plants on shallower soils tend to decrease in height relative to those on deeper soil (Harikrishna et al., 2005;Tardella et al., 2017).
One way to study the effects of the local environment on dispersal traits is to select environments that exist in extremes.For example, grassland habitats tend to have relatively deep soil, but a subset of herbaceous species that live in grasslands can also live in similar habitats with much shallower soils (e.g., alvars, glades, serpentine, rocky outcrops) (Hamilton & Eckert, 2007;Nelson, 1987;Visioli et al., 2019).By comparing dispersal potential of species that can live in both habitat types, we can determine how the suite of local environmental conditions that drive these habitat differences alter plant movement.
The simultaneous impacts of both climate and local environmental differences driven by habitat type on dispersal could result in either concomitant or interactive outcomes.When developing predictions for these interactive effects, results are likely to be highly dependent on the species in question, its evolutionary history, and its current environmental conditions (Beckman et al., 2020).If a species' dispersal ability responds in concert with respect to both habitat type and increasing temperatures, then you would expect the species to either increase or decrease its dispersal ability as these factors change.However, more complicated outcomes can arise when interactive effects occur, as Delattre et al. (2013) observed in a species of butterfly.When a species' dispersal ability responds differently to climatic factors in different types of environments, the resulting interactive effects can lead to more nuanced environmentspecific responses.Interactive effects are nearly impossible to tease apart without considering both factors directly and simultaneously, especially when interactions are nonadditive (Galic et al., 2018;Harpole et al., 2011).Thus, it is imperative to quantify the relationship between climate and habitat type simultaneously on dispersal ability in order to predict changes to these selective pressures (Fahrig, 2017;Parmesan & Yohe, 2003).
Here, we examine how climate and habitat type interact to impact dispersal ability using a species that spans a broad climatic gradient across prairie and alvar habitat types, Geum triflorum, Prairie Smoke (Rohrer, 1993).We examine contemporary differences in population-level dispersal trait values and use these to estimate changes in dispersal potential.Wind-dispersed plant species, like G. triflorum, have evolved multiple mechanisms that promote movement by wind, including structures such as plumes, wings, or samaras that increase time aloft and in turn increase dispersal distances (Greene & Johnson, 1990;Lentink et al., 2009;Varshney et al., 2012).
Specifically, we aim to determine if climate (specifically, the number of growing degree days above 5°C) and habitat type (either isolated alvars with shallow soils or more continuous prairie habitats with deeper soils) have positive, negative, or interactive effects on contemporary dispersal trait variation including shape, mass, and falling speed that collectively influence dispersal potential.We measured dispersal traits from 100 G. triflorum populations across its range, including populations from both alvar and prairie habitats.Using these traits, we parameterized dispersal models to determine dispersal potential of individuals within each population.We found that climate and habitat type additively interact to influence dispersal trait variation, and in turn dispersal potential of G. triflorum.In prairie habitats, increasing the number of growing degree days increased dispersal ability, whereas in alvar habitats, increasing the number of growing degree days reduced dispersal ability.These results indicate the importance of considering the interaction between climate and the habitat type on dispersal potential and provide important hypotheses for future experiments to test the causal factors that drive a species' ability to spatially track shifting fitness optima under global change.

| Study species
Geum triflorum (Pursh), or Prairie Smoke (Rosaceae), is an herbaceous perennial distributed across much of northern North America (US and Canada) and in the southwestern United States and California (Gajewski, 1958;Rohrer, 1993).Inflorescences typically have three nodding flowers that become erect at seed set.The sessile infructescence contains multiple achenes, each connected to a densely wooly style that remains intact during dispersal (Figure 1a).This structure, hereafter termed "diaspore," includes both the achenes and style (Figure 1b-d).These styles likely promote wind dispersal by increasing the time-aloft for diaspores following release from the maternal plant (Greene & Johnson, 1989, 1990).

| Sampling
We collected individual seed heads from populations of G. triflorum in 2002, 2003, 2015, and 2016.These populations were defined as distinct groups of plants separated from other populations by at least 1 km.Individual seed heads were collected from between 40 and 150 plants along a 100 m transect in 2002, 2003, and 2015 and along a 1000 m transect in 2016.See Hamilton and Eckert (2007) for detailed sampling methods.In total, we sampled 60 continuous prairie populations and 40 isolated alvar populations (ecoregional types described in Section 2.3 below, Figure 2).

| Habitat type
To determine how differences in habitat type alter dispersal traits, we sampled G. triflorum populations across its range throughout the midwestern prairies of North America, and across disjunct populations geographically isolated on limestone barrens known as alvar habitats, which are distributed throughout the Great Lakes region and into Manitoba, Canada.The species occurs in four of seven alvar regions across the Great Lakes (Catling & Brownell, 1995;Reschke et al., 1999); the Napanee Plain, the Carden Plain, Western New York, and Manitoulin Island; as well as alvar habitats on Drummond Island, Michigan, and northern In general, prairies and alvars differ in both abiotic and biotic conditions.Their edaphic environments differ with alvars having thin layers of soil over limestone and exposed areas of bare rock (Belcher et al., 1992;Catling & Brownell, 1995;Hamilton & Eckert, 2007), contrasting with prairies which tend to have deep soils which allow for deeper rooting systems (Anderson et al., 2006;Volk et al., 2022).The plant communities on alvars and prairies are also different.Alvars host a large number of species that are disjunct from their core distributions (northern, southern, western disjuncts), in addition to some endemics and persist as islands of open habitat within a broader matrix of conifer-dominated boreal forest (Brownell & Riley, 2000;Catling & Brownell, 1995).In contrast, prairies are largely open, and comprised of mixed, short, or tallgrass systems that are subject to climatic extremes (Samson & Knopf, 1994).In general, local densities of G. triflorum tend to be slightly higher in alvars than prairies.
Historic connectivity also varies between prairie and alvar populations (Hamilton & Eckert, 2007), which has implications for the evolution of dispersal traits.Prairie populations have historically experienced higher levels of genetic connectivity (Hamilton & Eckert, 2007), but extreme anthropogenic fragmentation across its range over the last century (Lark et al., 2018;Samson & Knopf, 1994;Wright & Wimberly, 2013) has led to increased isolation (Sullivan et al., 2021;Wimberly et al., 2018).In contrast, historically isolated alvar populations are not only disjunct from the main contiguous range across the midwestern prairies but also are isolated from each other.Grassland species likely colonized alvar habitats (Hamilton & Eckert, 2007), however, it is likely that subsequent range contractions isolated grassland species, including G. triflorum, on alvar habitats (Catling & Brownell, 1995;Hamilton & Eckert, 2007).Genetic data indicate that alvar populations have a subset of the genetic variation found in prairie populations, and within the same geographic distance, alvar population pairs are significantly more genetically differentiated from each other than prairie population pairs (Hamilton & Eckert, 2007).Thus, contemporary differentiation between prairie and alvar populations likely reflects a combination of colonization history and contemporary gene flow, stochastic processes associated with shifts in population demography, and changing selective processes leading to adaptation across regional environments.

| Climate
To determine how climate influences dispersal traits of G. triflorum, we used year of collection, latitude, longitude, and elevation from sampling locations as inputs into ClimateNA (Wang et al., 2016) to estimate climate variables associated with geographic provenance.
We estimated climatic variables associated with the year of seed collection to control for temporal variation in the maternal environment that may influence development across our multiple years of sampling, and because the climate associated with the year of collection represents the environmental conditions that corresponds with diaspore development.For each population, we were able to estimate annual climate variables including many related to precipitation and temperature.In order to reduce redundancy and account for correlation across climate variables, we performed a principal component analysis (PCA).We found temperature variables (number of growing degree days above 5°C, mean annual temperature, frost-free period, etc.) largely loaded on the PC1 axis and explained 51% of the variation, while variables associated with water availability (mean annual precipitation, climatic moisture deficit, annual heat moisture index, etc.) largely loaded on PC2 axis and explained 23.6% of the variation.To simplify our analyses, we described climate using the "number of growing degree days above 5°C" variable (hereafter referred to as DD5) as it loads highly on PC1 representing much of the temperature variability spanning our populations.Specifically, DD5 is defined as the sum of days when the mean temperature was above 5°C and reflects the time of "active growth" for Prairie Smoke populations (Thibault et al., 2020).
Furthermore, it is a biologically meaningful climate variable as it reflects the required heat sum associated with the onset of growth (Beaubien & Hamann, 2011).Development is predicted to be particularly sensitive to climate change (McGinn & Shepherd, 2003) as shifts in DD5 for spring perennials, such as G. triflorum, have substantial influence on the timing and duration of the growing season (Beaubien & Hamann, 2011;Kulbaba et al., 2023;Volk et al., 2022;Whittet et al., 2017).We note here that both prairie and alvar populations fell within a similar latitudinal range, therefore values of DD5 are largely independent of photoperiod (e.g., each habitat type on the whole had overlapping distributions of DD5).

| Dispersal trait measurements
To measure dispersal traits, we randomly selected three to five individual seed heads per population, each seed head representing one maternal family.We then measured three types of dispersal traits on all diaspores (Figure 1), including mass, morphology, and terminal velocity (or falling speed).Pooling five individual diaspore measures per maternal family, we first estimated total diaspore mass.We weighed both the achene and style separately to the nearest 0.0001 g.Following this, we examined morphological variation for each of five individual diaspores per maternal family.We photographed diaspores on a 5 mm 2 grid using a Leica DM2500 dissecting microscope for all morphological measurements.We placed a glass sheet atop each diaspore to uniformly flatten the dispersal structures.All measurements were made using ImageJ (Schneider et al., 2012).We measured the total length of the style and achene to the nearest 0.001 mm, and the achene area to the nearest 0.001 mm 2 .We measured the total length of the style and achene as the entire path length.We calculated achene area using the length and width measured at the longest and widest points, respectively.In addition, because diaspore length represented the path length of each seed, but sometimes seeds were folded, and not straight (Figure 1c vs. 1b), we also calculated diaspore shape index, which is a measure of 2D shape of each diaspore.This was calculated as an area, using the length of the longest dimension, and the perpendicular longest length.Finally, to measure terminal velocity, we dropped up to five individual intact diaspores per maternal family through a modified 10 cm wide PVC tube with two sets of light sensor arrays spaced 57 cm apart that record when the diaspores passed through both sets of arrays.We recorded the time it took the diaspores to cross this distance as the terminal velocity.For a full description of the terminal velocity measurement device and setup, see Sullivan et al. (2018).Diaspores were dropped from a height of 1 m to ensure they reached terminal velocity.For terminal velocity measurements we used data from 26 prairie populations and 17 alvar populations reflecting contemporary collections from 2015 to 2016.
In order to translate terminal velocity measurements into dispersal ability, we used the WALD model (Katul et al., 2005), which has been empirically validated for grassland species (Soons et al., 2004;Sullivan et al., 2018), to calculate the dispersal distances traveled by the farthest 1% of dispersers.To do this, we followed Sullivan We used a grand average of 7-day wind averages collected approximately every 4 days during the month of June in 2017 and 2018 as this is when the majority of G. triflorum diaspore dispersal occurs.We used these data to estimate wind parameters in prairie systems, and assumed the wind to be either equivalent between the two habitat types, or 50% of this average value for alvars, as these isolated habitats are surrounded by forest trees, which can substantially reduce wind effects (Damschen et al., 2014).Dispersal kernels were modeled for each individual diaspore within each habitat type, following which we extracted the predicted distance traveled if the diaspore achieved its long-distance dispersal potential (Higgins et al., 2008).

| Statistical analysis
To determine how climate and habitat type (alvar vs. prairie) interact to influence dispersal traits of G. triflorum, we explored our response variables with mixed effect models.We ran three separate models, one for each of the three categories of dispersal traits, including mass, morphology, and terminal velocity.For these dispersal trait models, all trait variables (response variables) were transformed using a square root transformation to meet the assumption of normality.
For these models, the fixed effects included the interaction between habitat type (prairie vs. alvar) and the log DD5 estimated from the growing season in which diaspores were collected.Our random intercepts included family nested within population (except for the mass model that only had one value per family, thus we simply included a random effect for population), and an additional random intercept term for year of data collection.We note that year effects explain about 20% of the variation for these models, but opt to leave this in the random effects because it is not the main focus of our study.This appears to correlate strongly with precipitation patterns.
Additionally, to determine the effect of climate and habitat continuity on dispersal potential, we analyzed the log of the distance traveled by the farthest 1% of dispersing individuals using another mixed effects model.Here, our fixed effects were the interaction between habitat type and log DD5.Our random effects included an intercept of family nested within population, and a year intercept.
We used R v4.2.0 (R Core Team, 2022) to conduct mixed effect models with the lmer() function in the lme4 package (Bates et al., 2015), and used the lmerTest package (Kuznetsova et al., 2014) to extract p-values.We also used the MuMIn package (Barton, 2018) to determine the contributions of the fixed and random effects using r 2 values.Data and code for analyses and figure creation can be found at Dryad: https:// doi.org/ 10. 5061/ dryad.3bk3j 9kt0.

| Mass measurements
We found a significant interaction between habitat type (alvar vs. prairie) and climate for total diaspore mass (Table 1A, p = .021),where fixed effects explained 4.6% of the variance, and fixed and random effects together explained 65.6% of the variance.As the length of the growing season increases, prairie diaspores become heavier, while alvar diaspores become lighter (Figure 3a).

| Morphology measurements
We found a significant interaction between habitat type (alvar vs. prairie) and climate on total diaspore length (seed plus style length) (Table 1B, p = .003),with fixed effects explaining 7.74% of the variation, and fixed and random effects together explaining 80.8% of the variation.This interaction suggests as growing season increases, diaspore length increases in prairies, but changes little in alvars (Figure 3b).We did not find a significant effect of habitat type or climate on diaspore area or diaspore shape index.

| Terminal velocity measurements
We also observed a significant interaction between habitat type (alvar vs. prairie) and climate for terminal velocity of the diaspores (Table 1C 3c).
We note for these three traits, the fixed effects explain a relatively small amount of variance, with much left to be explained.This is due in part to large sample size, and much of the variance being explained by the maternal effects and site and year differences.
We encourage interpretation of our fixed effects with appropriate caution.

| Estimated dispersal distances
We found that the interaction between habitat type (alvar vs. prairie) and climate significantly altered potential long-distance dispersal of G. triflorum (Table 1D,E).When wind conditions were equal between prairie and alvar habitats, we found a significant interaction between climate and habitat type (t = 3.91, p = .001)with fixed effects explaining 6.7% of the variance, and fixed and random effects together explaining 39.6% of the variance.Longer growing seasons predicts increased long-distance dispersal potential in the prairies, and reduced Relationship between diaspore traits and degree days above 5°C (DD5) in both prairie (light gray) and alvar (dark gray) regions.Total diaspore mass (a) showed a significant interaction between habitat type (prairie vs. alvar) and climate-as the growing season gets longer (increased DD5), diaspore mass increased in prairies, while alvar diaspore mass decreased slightly.Diaspore length (b) shows a significant interaction between habitat continuity and climate.As the season gets longer (increased DD5), in prairies diaspores get longer, and in alvars, there is little change.Terminal velocity (c) showed a strong interaction between habitat continuity and climate.As the length of the growing season increased (increased DD5), in alvars the terminal velocity of a diaspore increased, in prairies, the terminal velocity decreased.This indicates that diaspores have the potential to travel further distances in prairies than in alvars, in longer seasons, but travel further in alvars when the season is shorter.

F I G U R E 4
When using the WALD model to estimate dispersal distances for Geum triflorum individuals, we found a significant interaction between habitat type (prairie vs. alvar) and climate.As the length of the growing season increased (increased degree days above 5°C), in alvars (dark gray) the potential long-distance dispersal ability (the distance traveled by the farthest 1%) of a diaspore increased, in prairies (light gray) the long-distance dispersal ability increased.We ran these models under (a) equal wind conditions between prairies and alvars, and (b) a 50% reduction in wind speeds in alvar habitats compared with prairies.
long-distance dispersal potential in the alvars (Figure 4a).When wind conditions were 50% slower in alvar habitats, which is a reasonable assumption given alvars are predicted to have lower average wind speeds relative to prairie environments (Damschen et al., 2014;Schaefer & Larson, 1997), we found a similar significant interaction between climate and habitat type (t = 2.74, p = .012),with fixed effects explaining 43.2% of the variance, and fixed and random effects together explaining 66.0% of the variance.While the results are qualitatively similar, the differences in dispersal potential are much larger under the 50% wind reduction scenario (Figure 4b).

| DISCUSSION
Here we demonstrate that climatic conditions and habitat type can interact to influence dispersal trait variation and potential dispersal ability.We found that with the accumulation of growing degree days, Geum triflorum exhibited increased dispersal potential in prairie habitats.In contrast, isolated alvar populations exhibited reduced dispersal potential as the number of growing degree days increased.Similar to the results of Delattre et al. (2013), these observational data suggest the relationship between movement, habitat type, and climate can be complex and interdependent.
These results have the potential to predict the capacity by which G. triflorum will be able to track shifting fitness optima in rapidly changing environments via dispersal across varying environmental conditions (Davis & Shaw, 2001;Kokko & López-Sepulcre, 2006).
We encourage experimentally-based examination of the interplay between climate and habitat type and its impact on dispersal potential across species, especially those of conservation concern, in order to predict shifts in gene flow and genetic connectivity across dynamic landscapes.
Our results suggest variance in dispersal potential is at least partly driven by the environment.For populations on alvars, which experience predictable seasonal variation from flooding to desiccation, in a longer growing season (e.g., more growing degree days above 5°C), limited access to water as the season progresses may impact development (Hamilton & Eckert, 2007;Volk et al., 2022).We find alvar populations of G. triflorum use shorter seasons with conditions that promote less water stress to produce seeds that can move farther away from their natal location.Whereas a longer growing season, which is more prone to water-limitation on alvar habitats (increased DD5) leads to shorter dispersal distances with seeds remaining in their natal locations.However, in prairie populations where soils are deeper and there is more access to water, the growing season may extend as degree days increase.Here, G. triflorum has more time for active growth (Kulbaba et al., 2023) and can send its seeds farther.Recent evidence examining the phenological response of G. triflorum reflects these trends, where traits associated with initial establishment are under strong genetic control, but later life-history stages are under stronger environmental control (Kulbaba et al., 2023).
The colonization history of isolated alvar habitats suggests that the strength and direction of selection for dispersal traits for grassland species, such as G. triflorum, isolated in alvar habitats could have changed over time (Catling & Brownell, 1995;Hamilton & Eckert, 2007).Previous research suggests that alvar habitats were likely colonized by grassland species during the warming Hypsithermal period ~5000 YBP as grasslands expanded (Hamilton & Eckert, 2007).However, following a cooling period, subsequent range contractions likely isolated a number of grassland species, including G. triflorum, on alvar habitats (Catling & Brownell, 1995;Hamilton & Eckert, 2007).While traits associated with long-distance dispersal may have been selected for during initial colonization of alvar habitats, following isolation selection may act against traits associated with dispersal, as our results suggest.
Furthermore, ecological specialization and local adaptation across alvar habitats may limit further gene flow, strengthening selection against dispersal (Van Den Elzen et al., 2016;Yoko et al., 2020).In order to determine the potential strength of selection in these two different habitats, reciprocal transplants would be required to tease apart potential confounding effect of having different soil conditions between these two habitats, as well as the respective role of the environmental variation from genetic differences, which reflect both heritable genetic differences and maternal effects, both of which have been shown to alter dispersal traits (Donohue, 1998;Galloway, 2005;Jacobs & Lesmeister, 2012;Yoko et al., 2020).
While we have captured a snapshot of the relationship between environment and dispersal trait variation, we have observed that much of the variation can be explained by the maternal line, as well as variation caused by the environment at each site (large proportion of variation explained by the random effects).Temporal monitoring of trait variation within reciprocal transplant experiments will aid in understanding the role that plasticity and maternal effects play in maintaining variation in dispersal traits in response to seasonal environmental change.
There are substantial eco-evolutionary consequences for differences observed in contemporary dispersal traits, such as those observed in G. triflorum.When considering the ability for diaspores to move, terminal velocity is critical (Wilson, 2000).This composite trait takes into account mass, morphology, and physical structures, such as hairs, to create differences in the length of time a diaspore can stay aloft in a column of air (Greene & Johnson, 1990;Matlack, 1987;Platt & Weis, 1977;Sheldon & Burrows, 1973).
Terminal velocity plays an important role in a species' response to habitat fragmentation (Schleicher et al., 2011), and indirectly influences gene flow and the maintenance of connectivity across populations.In addition, we found that climate and habitat type altered another important dispersal trait, achene mass.This trait plays a role in establishment once individuals have dispersed, as larger propagules tend to have a higher survival probability during establishment (Moles & Westoby, 2004;Skarpaas et al., 2011).We find that achene mass and terminal velocity exhibit similar trends, as achene mass increases with increasing growing degree days in prairie habitats, but only slightly decreases with increased growing degree days in alvar habitats.When combined with terminal velocity, our results indicate that as climates warm and the number of growing degree days increases, G. triflorum populations in prairie habitats have the potential to disperse farther and may have an increased establishment probability, while alvar populations exhibit decreased dispersal and establishment potential.
In the landscapes over which G. triflorum occurs (i.e., continuous prairies vs. disjunct alvars), the spatial scale of habitat availability is highly relevant to selection on dispersal ability.For alvar versus prairie habitats, the frequency and location of suitable habitats may impact dispersal trait variation.According to the theory of island biogeography (MacArthur & Wilson, 1967), populations in isolated habitats may select against dispersal, particularly where long-distance dispersal leads to reduced fitness as organisms disperse beyond the range of suitable habitats (Reluga & Shaw, 2015;Schenk, 2013;Shaw et al., 2019).Disjunct alvar habitats resemble oceanic islands, existing as open, isolated limestone barrens situated within a matrix of boreal forest.Similar to gypsum outcrops described by Van Den Elzen et al. (2016) and in increasingly patchy landscapes in Spain as described Riba et al. (2009), the low frequency of disjunct alvar habitats relative to more continuous prairies will lead to strong selection against dispersal within alvar populations, as propagules that disperse longer distances likely land in unsuitable habitat.While selection may have shifted to favor reduced dispersal for isolated alvar populations, the same may not be true of prairie populations.
Relatively continuous prairie habitats increase the likelihood that dispersing propagules will land in suitable habitats, contributing to selection for increased dispersal (Travis & Dytham, 1999).However, previous and ongoing agricultural conversion may increase the scale of isolation for populations within a matrix of inhospitable environments (Lark et al., 2018;Wimberly et al., 2018;Wright & Wimberly, 2013).Thus, increasing fragmentation across prairie environments has the potential to shift the direction of selection for dispersal traits in these historically continuous habitats.Our study remains an observational, spatially implicit examination of how dispersal traits change in relation to the environment.We encourage future work to explore how these altered traits might influence spatial recruitment patterns in open habitat patches within the landscape (Platt & Weis, 1977).
Climatic variation and shifting habitat suitability have the potential to rapidly alter dispersal potential, impacting predictions for connectivity and species' range shifts in response to global change (Kubisch et al., 2013).Where growing seasons are predicted to change, region-specific dispersal potential in G. triflorum may become further reinforced (McGinn & Shepherd, 2003;Pryor et al., 2013;Wuebbles & Hayhoe, 2004).This may impact the evolutionary potential of populations, particularly those isolated alvar populations at the periphery of a species' range where reduced connectivity influences the maintenance of genetic variation needed to adapt to change (Jump & Peñuelas, 2005).Grasslands tend to be dominated by plants that disperse by wind (Lorts et al., 2008;Packard & Mutel, 1997).As climates warm, wind speed and turbulence patterns are expected to increase in the prairie region (Kling & Ackerly, 2020), fruiting head and (b-d) diaspores.Each diaspore contains an achene that is connected to a densely wooly style that remains intact at the time of dispersal.These hairs help slow the rate of falling and thus help increase the dispersal ability of these diaspores.Panels b-d represent the range of diaspore shapes and sizes.Manitoba.G. triflorum's prairie range extends from west of the Great Lakes across the Rocky Mountains, covering much of the Great Plains of Canada and the United States.

F
I G U R E 2 Geum triflorum populations sampled for the study across the United States and Canada.Yellow dots indicate prairie populations, and red dots indicate alvar populations.All populations are within a similar range of DD5 (number of growing degree days over 5°C).
et al. (2018) and used plant traits including height at diaspore release and terminal velocity to parameterize the estimated dispersal kernel for each diaspore.We estimated pooled height at diaspore release for each habitat type as the average height from 53 prairie and 18 alvar flowering stems of G. triflorum (measured from rosette to base of sepals) from herbaria specimens provided by the University of Minnesota (MIN), University of Manitoba (WIN), the Canadian Museum of Nature (CAN), and the Agriculture and Agri-Food Canada National Collection of Vascular Plants (DAO).Analysis of variance indicated no difference in height between plants in the alvars and prairies (F 1 = 0.461, p = .499).We estimated the canopy height to be 0.2 m for both regions at the time of G. triflorum seeding, as this is early in the season and many plants are still short.Finally, we parameterized wind values from weather station readings near Moorhead, MN.
could further enhance dispersal in these more open, continuous systems.While our study is observational in nature, we encourage future studies to control for the effects of climate on plants from isolated and continuous habitats in order to parse out how dispersal is (or is not) changing.Understanding a species' ability to sustain an evolutionary response to changing conditions is necessary for the conservation of plant populations, and requires an evaluation of the interaction between climate and landscape structure to establish predictions for species distributions and connectivity under global change.Conceptualization (equal); data curation (equal); formal analysis (lead); investigation (equal); methodology (lead); project administration (equal); resources (equal); supervision (equal); visualization (lead); writing -original draft (lead); writing -review and editing (equal).Zoe M. Portlas: Investigation (equal); writingreview and editing (equal).Kelsey M. Jaeger: Investigation (equal); writing -review and editing (equal).Mercedes Hoffner: Investigation (equal); writing -review and editing (equal).Jill A. Hamilton: Conceptualization (equal); data curation (equal); formal analysis (supporting); funding acquisition (equal); investigation (equal); methodology (lead); project administration (equal); resources (lead); supervision (equal); writing -review and editing (equal).O PEN R E S E A RCH BA D G E SThis article has earned Open Data and Open Materials badges.Data and materials are available at data and code are available at GitHub during the review process: https:// github.com/ LLSul livan/ GeumD ispersal.Upon acceptance data and code will also be permanently stored at Dryad.https:// github.com/ LLSul livan/ GeumD ispersal (see above for plan for long term data and code storage).
, p = .006),with fixed effects explaining 8.9% of the variance, Bold values indicate p < 0.05 considered as statistically significant.TA B L E 1 Mixed effects model resultsfor (A) diaspore mass, (B) total diaspore length, (C) terminal velocity, and (D) longdistance dispersal potential.